This invention relates to polyisocyanates of the diphenyl methane series in which isocyanate groups in the 3,4'- and, optionally, in the 3,2'-positions are present. This invention also relates to processes for the production of such polyisocyanates.
Difunctional diisocyanato-toluene isomers (TDI) and difunctional diisocyanato-diphenyl methane isomers (particularly, 2,4'- and/or 4,4'-diisocyanato-diphenyl methane) are among the most important polyisocyanates in polyurethane chemistry, both technically and economically. Mixtures of higher polyisocyanates of the diphenyl methane series which may be obtained by the phosgenation of aniline/formaldehyde condensates are particularly important materials. These higher polyisocyanate mixtures of the diphenyl methane series, which on a statistical average have an NCO functionality of more than two, are particularly important as starting materials for the production of polyurethane foams. However, conventional mixtures of this type are disadvantageous because they are mixtures of distillable polyisocyanates of low functionality with non-distillable homologues of relatively high functionality. Any increase in NCO functionality of the entire mixture is obtained at the expense of an increase in the molecular size of the high functionality polyisocyanates and consequently an increase in the viscosity of the mixture. Another disadvantage of conventional diphenyl methane polyisocyanate mixtures of relatively high functionality is their tendency to undergo partial crystallization at low temperatures. Such crystallization makes it necessary to heat or chemically treat the mixture to liquefy it prior to use. Additionally, substantially pure polyisocyanates having an NCO functionality of three may be produced with considerable difficulty by the traditional methods. Although it is theoretically possible to separate trifunctional polyisocyanates from phosgenation products by distillation, considerable quantities of distillation residues accumulate which contain both polyisocyanates having a functionality of more than three and secondary products having carbodiimide and isocyanurate groups. Because of the high viscosity of such residues and the presence of undesirable secondary products, these distillation residues would be of limited value in the production of high-quality polyurethane plastics.